Distributed acoustic sensor (DAS) systems and distributed temperature sensor (DTS) systems are known in the art. Both DAS and DTS systems are commercially available from the present applicant, amongst others, referred to as the Silixa iDAS system, and the Silixa Ultima DTS system. Further details of both systems were available before the priority date at http://www.silixa.com/technology/idas/, and http://www.silixa.com/technology/its/. In addition, further technical details of the operation of the Silixa iDAS system can be found in our earlier patent applications WO2010/136809 and WO2010/136810, any details of which that are necessary for understanding the present invention being incorporated herein by reference.
The acoustic sensing and the dynamic strain spatial resolution of the known Silixa iDAS system is of the order of 1 m-5 m, in that a separate acoustic signal can be resolved for every 1 m-5 m or so of fiber. Thus, such a DAS provides the equivalent sensing capacity as if an array of synchronised microphones were to be placed approximately at every 1 m-5 m intervals. Whilst presently providing class-leading performance that is suitable for many applications, in some potential applications it would be beneficial if even higher spatial resolution could be provided, for example of 5 cm to 50 cm, or even measured in mm.
One known technique for trying to improve the spatial sensing resolution is illustrated in FIG. 17 of WO2010/136810. Here, an optical fibre sensor is being used to monitor a pipeline, and the fibre is installed running along the pipe. In order to try and improve the spatial resolution of the sensor at certain points along the pipe, the fibre has in places been helically wrapped around the actual pipe to be monitored. The effect of this is that, because the DAS system can effectively resolve an individual acoustic sensing point say approximately every 5 m along the length of the fibre (although in some cases resolution can be as high as 1 m), because the fibre is wrapped around the pipe, the sensing resolution in the direction along the pipe is substantially increased, essentially by a factor dependent on the circumference of the pipe. For example, as a simple example, assume the circumference of the pipe was also 5 m, then there would be an effective acoustic sensor measurement taken for substantially every turn around the pipe. The longitudinal sensor resolution along the pipe is therefore increased to the pitch of the helical winding of the fiber around the pipe. As another example assume the circumference of the pipe is 10 cm and we wrapped the fibre sensor around the pipe multiple turns, then there would be an effective acoustic sensor measurement taken for substantially every 5 cm to 50 cm section of pipes. In addition, the acoustic and dynamic strain sensitivity of fibre is enhanced for flow and seismic measurements in the wrapped section.
Such a solution is acceptable in some scenarios, for example where a section of the fibre can be directly wrapped around a section of a pipe or flow lines. However, it is more desirable and in some scenarios, such as wellbore installations, for the fiber to be simply laid within or next to the object or area to be monitored. Moreover, physically winding the fiber around the object to be monitored may not be possible, or be prohibitively expensive.
In a related field, optical fiber technology has also advanced in recent years, and in particular in the field of bend-insensitive (BI) fibers. These are optical fibers that permit a high degree of bending, in some cases with a bend radius as low as 5 mm, without large optical power loss from the core. The ITU has issued a number of standards for BI fiber, and in particular the ITU-T G.657 family of standards. Of these, BI fiber compliant with ITU-T G.675.A.3 can tolerate a bend radius of down to 5 mm, with a loss of 0.15 dB/turn at such a radius. A larger radius typically significantly reduces the loss per turn. For example, for BI cable in accordance with the ITU-T G.675.A.2 standard (which permits a minimum bend radius of 7.5 mm), the loss for 10 turns at 1550 nm with 15 mm bend radius is only 0.03 dB, whereas for a 7.5 mm bend radius the loss for 1 turn is 0.5 dB. The performance of BI fiber is being improved constantly, with lower losses for smaller bend radii. Example commercially available BI fibers are the ClearCurve® family of fibers, available from Corning Incorporated, of Corning, N.Y.
Finally, US 2013/0094798 describes a fiber optic cable having an outer protective coating surrounding an optical fiber. The optical fiber itself is described as having a helical core, which includes a plurality of sensors typically equally spaced along the helical core.